diff --git a/module/Core/src/Exception/ShortCodeCannotBeRegeneratedException.php b/module/Core/src/Exception/ShortCodeCannotBeRegeneratedException.php
index e1df3068..fad5805d 100644
--- a/module/Core/src/Exception/ShortCodeCannotBeRegeneratedException.php
+++ b/module/Core/src/Exception/ShortCodeCannotBeRegeneratedException.php
@@ -6,24 +6,13 @@ namespace Shlinkio\Shlink\Core\Exception;
 
 class ShortCodeCannotBeRegeneratedException extends RuntimeException
 {
-    /** @var @bool */
-    private $reasonIsSlug = false;
-
     public static function forShortUrlWithCustomSlug(): self
     {
-        $e = new self('The short code cannot be regenerated on ShortUrls where a custom slug was provided.');
-        $e->reasonIsSlug = true;
-
-        return $e;
+        return new self('The short code cannot be regenerated on ShortUrls where a custom slug was provided.');
     }
 
     public static function forShortUrlAlreadyPersisted(): self
     {
         return new self('The short code can be regenerated only on new ShortUrls which have not been persisted yet.');
     }
-
-    public function reasonIsSlug(): bool
-    {
-        return $this->reasonIsSlug;
-    }
 }
diff --git a/module/Core/test/Entity/ShortUrlTest.php b/module/Core/test/Entity/ShortUrlTest.php
new file mode 100644
index 00000000..98404ca1
--- /dev/null
+++ b/module/Core/test/Entity/ShortUrlTest.php
@@ -0,0 +1,51 @@
+<?php
+
+declare(strict_types=1);
+
+namespace ShlinkioTest\Shlink\Core\Entity;
+
+use PHPUnit\Framework\TestCase;
+use Shlinkio\Shlink\Core\Entity\ShortUrl;
+use Shlinkio\Shlink\Core\Exception\ShortCodeCannotBeRegeneratedException;
+use Shlinkio\Shlink\Core\Model\ShortUrlMeta;
+
+class ShortUrlTest extends TestCase
+{
+    /**
+     * @test
+     * @dataProvider provideInvalidShortUrls
+     */
+    public function regenerateShortCodeThrowsExceptionIfStateIsInvalid(
+        ShortUrl $shortUrl,
+        string $expectedMessage
+    ): void {
+        $this->expectException(ShortCodeCannotBeRegeneratedException::class);
+        $this->expectExceptionMessage($expectedMessage);
+
+        $shortUrl->regenerateShortCode();
+    }
+
+    public function provideInvalidShortUrls(): iterable
+    {
+        yield 'with custom slug' => [
+            new ShortUrl('', ShortUrlMeta::createFromRawData(['customSlug' => 'custom-slug'])),
+            'The short code cannot be regenerated on ShortUrls where a custom slug was provided.',
+        ];
+        yield 'already persisted' => [
+            (new ShortUrl(''))->setId('1'),
+            'The short code can be regenerated only on new ShortUrls which have not been persisted yet.',
+        ];
+    }
+
+    /** @test */
+    public function regenerateShortCodeProperlyChangesTheValueOnValidShortUrls(): void
+    {
+        $shortUrl = new ShortUrl('');
+        $firstShortCode = $shortUrl->getShortCode();
+
+        $shortUrl->regenerateShortCode();
+        $secondShortCode = $shortUrl->getShortCode();
+
+        $this->assertNotEquals($firstShortCode, $secondShortCode);
+    }
+}
diff --git a/module/Core/test/Service/UrlShortenerTest.php b/module/Core/test/Service/UrlShortenerTest.php
index 0185ef1a..78e61a6a 100644
--- a/module/Core/test/Service/UrlShortenerTest.php
+++ b/module/Core/test/Service/UrlShortenerTest.php
@@ -81,6 +81,32 @@ class UrlShortenerTest extends TestCase
         $this->assertEquals('http://foobar.com/12345/hello?foo=bar', $shortUrl->getLongUrl());
     }
 
+    /** @test */
+    public function shortCodeIsRegeneratedIfAlreadyInUse(): void
+    {
+        $callIndex = 0;
+        $expectedCalls = 3;
+        $repo = $this->prophesize(ShortUrlRepository::class);
+        $shortCodeIsInUse = $repo->shortCodeIsInUse(Argument::cetera())->will(
+            function () use (&$callIndex, $expectedCalls) {
+                $callIndex++;
+                return $callIndex < $expectedCalls;
+            }
+        );
+        $repo->findBy(Argument::cetera())->willReturn([]);
+        $getRepo = $this->em->getRepository(ShortUrl::class)->willReturn($repo->reveal());
+
+        $shortUrl = $this->urlShortener->urlToShortCode(
+            new Uri('http://foobar.com/12345/hello?foo=bar'),
+            [],
+            ShortUrlMeta::createEmpty()
+        );
+
+        $this->assertEquals('http://foobar.com/12345/hello?foo=bar', $shortUrl->getLongUrl());
+        $getRepo->shouldBeCalledTimes($expectedCalls);
+        $shortCodeIsInUse->shouldBeCalledTimes($expectedCalls);
+    }
+
     /** @test */
     public function transactionIsRolledBackAndExceptionRethrownWhenExceptionIsThrown(): void
     {
@@ -190,6 +216,12 @@ class UrlShortenerTest extends TestCase
             ShortUrlMeta::createFromRawData(['findIfExists' => true, 'validUntil' => Chronos::parse('2017-01-01')]),
             new ShortUrl($url, ShortUrlMeta::createFromRawData(['validUntil' => Chronos::parse('2017-01-01')])),
         ];
+        yield [
+            $url,
+            [],
+            ShortUrlMeta::createFromRawData(['findIfExists' => true, 'domain' => 'example.com']),
+            new ShortUrl($url, ShortUrlMeta::createFromRawData(['domain' => 'example.com'])),
+        ];
         yield [
             $url,
             ['baz', 'foo', 'bar'],